1. Field of the Invention
The present invention relates to a method for the hyperintense display of areas in the vicinity of dipole fields, a magnetic resonance scanner, a computer program and an electronically readable data medium for said program.
2. Description of the Prior Art
The magnetic resonance (MR) technology is a known image modality, with which images of the interior of an object to be examined can be generated. Stated simply, the object to be examined is positioned in a magnetic resonance apparatus in a strong static, homogenous basic magnetic field, also referred to as a B0 field, having field strengths of 0.2 tesla to 7 tesla and higher, such that nuclear spins in the object orient themselves along the basic magnetic field. High frequency excitation pulses (RF pulses) are directed into the object to be examined in order to trigger nuclear magnetic resonances. The triggered resonance signals are measured (detected), and based on such measurements, MR images are reconstructed or spectroscopic data are determined. For spatially encoding the measurement data, rapidly activated magnetic gradient fields are superimposed on the basic magnetic field. The recorded measurement data are digitalized and stored as complex number values in a k-space matrix in a memory. An associated MR image can be reconstructed from the k-space matrix populated with such values, e.g. by means of a multi-dimensional Fourier transformation.
For various medical applications and/or investigations, probes or particles may be used in which magnetically active substances are integrated. In this manner, it is possible for these particles or probes to be verified and localized by means of MR technology.
The magnetically active substances, however, induce a dipole field in their vicinity, which disturbs the otherwise ideally homogenous main magnetic field of the magnetic resonance scanner. This leads to signal losses with gradient echo (GRE) sequences and to distortions (susceptibility artifacts) in spin echo (SE) sequences. The resulting image areas, known as hypointense, image areas, cannot be traced back unambiguously, as a source, to the particles or probes having the integrated, magnetically active substances, because numerous other causes exist for a hypointensity of this sort. For this reason, MR methods are frequently used for verifying and localization of particles or probes of this type that generate a hyperintense contrast, meaning a brighter signal in the vicinity of the dipole field. Methods of this type exploit the fact that the magnetic dipole field of the particle or probe causes magnetic field gradients as well as changes to the proton Larmor frequencies in the immediate vicinity of the disturbance or impediment in order to localize and/or verify the particle or probe.
There are basically two known, different approaches to hyperintense measurement of magnetic impediment particles, i.e. particles having, for example, integrated magnetically active substances. One of these approaches exploits the gradients that exist as a result of the induced dipole field, and the other approach exploits the displacement of the Larmor frequencies in the region of the dipole field.
One example of the first approach noted above is described in Seppenwoolde et al., “Passive Tracking Exploiting Local Signal Conservation: The White Marker Phenomenon,” Magnetic Resonance in Medicine 50:784-790 (2003), which presents a GRE based method that exploits the additional dephasing of the spin in the impact area of the disturbance caused by the disturbance gradients for its localization.
An example of the second approach is described in Stuber et al., “Positive Contrast Visualization of Iron Oxide-Labeled Stem Cells using Inversion-Recovery With ON-Resonant Water Suppression (IRON),” Magnetic Resonance in Medicine 58:1072-1077 (2007). With the spectral method presented therein, an expansion of the water frequencies in the impact area of the impurities, caused by the induced dipole field, is exploited in order to localize said impurities. For this purpose, the frequencies of the so-called fat and water peaks are suppressed by means of inversion, such that only signals from the expanded frequency areas are recorded. The signal suppression is combined with a conventional SE or GRE sequence, which allow only echo times that are significantly greater than a millisecond.
In the vicinity of magnetic impurities, e.g. magnetically active substances, long echo times of this type have the disadvantage that, through dephasing or susceptibility artifacts, many of the hyperintense signals are lost that actually should be depicted. Moreover, as a result, other areas are also hyperintensely depicted, lying within the impact area of other magnetic disturbances, such as air bubbles, for example. As a result, it is difficult to differentiate regions of other magnetic disturbances therein, if at all, from hyperintense areas in the vicinity of localizing probes or particles having integrated magnetically active substances.
According to the prior art, pulse sequences are known that enable a very short echo time. One example is the radial UTE sequence (“Ultrashort Echo Time”), as is described in the article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle,” Magn. Res. Med. 2007; 57; pages 74-81. With this sequence type, the start-up, and simultaneous data acquisition is initiated after a waiting period T_delay following a non-selective or slice-selective excitation of the gradients. The k-space trajectory (i.e., the path along which entries of raw data are made into k-space) scanned in this manner, following an excitation, proceeds radially from the k-space center outwardly. For this purpose, a conversion to a Cartesian k-space grid must first be carried out, prior to the reconstruction of the image data from the raw data recorded in k-space, by means of a Fourier transformation of the raw data, e.g. through regridding.
Another approach for enabling short echo times is to scan the k-space by points, in which the free induction decay (FID) is captured. A method of this sort is referred to as single point imaging, because for each RF excitation, basically only one raw data point in k-space is captured. An example of this type of method for single point imaging is the RASP method (“Rapid Signal Point Imaging,” O. Heid, M. Meimling, SMR, 3rd Annual Meeting, page 684, 1995). According to the RASP method, a raw data point is read out at a fixed point in time after the RF excitation at the “echo time” TE, the phase of which is encoded by gradients. The gradients are changed by means of the magnetic resonance scanner for each raw data point or measurement point, and in this manner k-space is scanned on a point-by-point basis, as is depicted in FIGS. 1a and 1b. 